1. Field of the Invention
The present invention relates to a device for cooling a fuel cell comprising a stack of unit cells with a cooling medium which is supplied from an external source and passes through cooling pipes disposed in cooling plates that are distributed in the cell stack.
2. Description of the Prior Art
One known cooling arrangement for cooling a fuel cell is disclosed in Japanese Laid-Open Patent Publication No. 61(1986)-147467, for example. The disclosed cooling arrangement includes cooling plates disposed in a stack of unit cells at spaced intervals each corresponding to several cells. An electrically nonconductive cooling medium (pure water) which is supplied from an external source passes through cooling pipes embedded in the respective cooling plates to remove reaction heat from the cell stack.
FIG. 5 of the accompanying drawings shows a cooling system incorporating such a known cooling arrangement for cooling a fuel cell. The fuel cell comrises stack 1 of unit cells and cooling plates disposed in the cell stack 1 at spaced intervals each corressponding to several cells. The cooling system includes metallic cooling pipes 2a embedded in the cooling plates 2. The cooling pipes 2a have inlet and outlet ends coupled through header pipes 3 to inlet and outlet main cooling pipes 4. Each of the cooling plates 2, which serve as conductive paths between the cells, is made of a material which is electrically conductive and has a high heat transfer rate. The cooling system also includes a cooling water circulating circuit 7 connected between the inlet and outlet main cooling pipes 4 , the cooling water circulating circuit 7 having a circulating pump 5 and a vapor-liquid separator 6. To the vapor-liquid separator 6, there are connected a vapor recovery pipe 7a and a cooling water replenishing pipe 7b. The fuel cell is stored in a container 8.
When the fuel cell is in operation, the circulating pump 5 is oparated to supply cooling water (pure water) 9 stored in the vapor-liquid separator 6 from the inlet main cooling pipe 4 through the header pipes 3 to the cooling pipes 2a in the cooling plates 2, thereby removing reaction heat generated in the cell stack 1. The cooling water 9 is kept at a temperature of about 180.degree. C. under a pressure of about 10 kg/cm.sup.2 in order to maintain the fuel cell at a operating temperature. Specifically, a portion of the cooling water is vaporized, and the latent heat generated when it is vaporized is employed to cool the cell stack 1.
As the fuel cell generates electricity, a potential difference is developed between adjacent two of the cooling plates 2 distributed in the cell stack 1. If the cooling pipes 2a were directly connected to the common main cooling pipes 4, then a short circuit having electrically conductive paths composed of the metallic pipes would be established across the cell stack 1. Heretofore, as shown in FIG. 6, insulating hoses 10 are connected between branch pipes 4a extending from the main cooling pipes 4 and the header pipes 3, so that potential differeces will be maintained between the cooling plates 2 and the main cooling pipes 4 which are grounded and hence at zero potential.
In FIG. 6, the cooling plates 2 in the cell stack 1 and the main cooling pipes 4 are electrically insulated by the insulating hoses 10 connected between the header pipes 3 and the branch pipes 4a with respect to each of the cooling plates 2. However the arrangement shown in FIG. 6 has the following maintenance problems:
As described above, the cooling water flowing through the cooling system is kept at a high temperature of about 180.degree. C. under a high pressure of about 10 kg/cm.sup.2. Terefore, the insulating hoses 10 connected to the pipes of the cooling system are required to be heat-resistant and pressure-resistant, as well as electically insulative, so that they can withstand the severe conditions in which they are used. It has been customary to construct the insulating hoses 10 of an electric insulating material such as fluoroplastlics and reinforce the hoses with outer pipes made of woven metallic wires. In order to prevent the high pressure cooling water from leaking from the insulating hoses 10, joint fittings capable of withstanding high pressure are attached to the opposite ends of each of the insulating hoses 10.
The insulating hoses of such a construction are however expensive to manufacture. Experience in long-term operaion of fuel cell indicates that the insulating hoses have a relatively short service life and often suffer degraded insulation and cause water leakage. The cooling system should therefore be kept in good condition through periodic replacement of the insulating hoses. However, the cost required by such periodic hose replacement is not negligible. Tere is a demand for certain measures which will solve this problem, as well as the reliability problem.